The devices are placed in "pay at the pump" card readers and can copy credit or debit card information.
Investigators say old style keypads are more easily compromised by skimmer devices.
To protect yourself, authorities say to look for newer key pads with individual buttons for each number, or pay inside.
Customers that used a credit or debit card at a 76 gas station in Rancho Cucamonga are being asked to check their bank statement immediately and check for discrepancies.
No information has been released as to how long the skimming devices found have been in place or how many consumers have been victimized, but authorities are asking customers to take extreme precautions.